The desalination of seawater is practiced in some coastal, arid regions of the world in association with electric power production. Waste heat from the power production process is available and used to drive the desalination process in these plants. In traditional practice, the waste heat is utilized for evaporation of saline water by Multi-Stage Flash (MSF) or Multi-Effect Distillation (MED) methods. Salts contained in the water are typically disposed of in liquid form by returning the residual, high-salinity water byproduct of the desalination process to the sea. In general, about half of the feed water, which typically has an initial salt (NaCl) concentration of 3–4.5%, will be upgraded and the remainder will typically be returned to the ocean.
Traditionally, desalination has been coupled to gas turbine power plants because of the high quality waste heat that is available to drive the evaporation process. Many modern power plants, however, use steam cycle technology or a combined cycle design in which a Brayton (or gas turbine) cycle is the topping cycle and a Rankine (steam) cycle is the bottoming cycle. These other types of power plant designs generally produce less waste heat and are generally not attractive for powering desalination of seawater. Because both the type of power plant to which the traditional desalination technology is applied and the quality of the water source are generally constant, only a limited number of technologies (e.g., MSF and MED) have been applied to desalination via waste heat utilization.
A problem with the MSF and MED technologies is that, even though the energy source is essentially without cost because the power plant provides waste heat to be used in the process, the capital costs for power plant construction are high and may be of the same order of magnitude of the capital cost for the power plant itself. These high costs for the desalination process stem from the process equipment needed for handling low pressure water vapor and the number of evaporator/condenser heat exchangers that are required. Another problem with the existing technique of desalinating water at power plants adjacent large open bodies of water, such as oceans, is that many arid regions of the world are not near coastal areas and there may be no acceptable source of surface water for purification. Further, because the standard desalination process is based on use of waste heat from power plants such as gas turbine (Brayton Cycle) power plants, and this type of plant is less efficient than steam plants (Rankine Cycle) or combined cycle plants, the existing desalination technologies used for sea water may not be directly applicable to use in future power plant designs or those power plants that are located away from any readily available source of open surface water.